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Editorial Team
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Asked: 2026-05-13T07:08:33+00:00

When is IPPROTO_UDP required? Is there ever a case where UDP is not the

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When is IPPROTO_UDP required?

Is there ever a case where UDP is not the default protocol for SOCK_DGRAM? (real cases, not hypothetical “it might be”, please”)

i.e., what are the situations where the following two lines would not produce identical behavior?

if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
if ((s=socket(AF_INET, SOCK_DGRAM, 0))==-1)
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1 Answer

  1. Editorial Team

    Given these declarations:

    tcp_socket = socket(AF_INET, SOCK_STREAM, 0);
udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
raw_socket = socket(AF_INET, SOCK_RAW, protocol);

    the ip(7) manual page in linux says:

    The only valid values for protocol are
    0 and IPPROTO_TCP for TCP sockets, and
    0 and IPPROTO_UDP for UDP sockets.
    For SOCK_RAW you may specify a valid
    IANA IP protocol defined in RFC 1700
    assigned numbers.

    Those two lines in your questions will always produce the same result.

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